1. Field of the Invention
The present invention relates to a process for the production of chlorinated polyvinyl chloride resin. More particularly, it relates to a process for producing chlorinated polyvinyl chlorides which process prevents the formation of floating resin in the step of chlorination and yields a chlorinated polyvinyl chloride resin excellent free flow property and in further, which is superior in initial coloring as well as heat stability at a time of molding with application of heat.
2. Prior Art
A chlorinated polyvinyl chloride resin (hereinafter referred to as "CPVC") can be obtained by chlorination of polyvinyl chloride resin (hereinafter referred to as "PVC"). Processes for chlorinating PVC suspended in an aqueous medium are known in the prior art and described, for example, in Japanese Laid Open Patent specifications Kokai Sho 58-103507 (1983) and Sho 64-6002 (1989).
When CPVC is chlorinated in aqueous suspension, the resins tend to float up, and because of this tendency, in the step of discharging the resins from the reaction vessel after completion of the reaction, the resins may partly remain in the vessel undischarged, or in the step of transportation of resin slurries, these may cause pump cavitation troubles. For these and other reasons, productivity is low.
On the other hand, CPVC is characterized by its higher heat resistance as compared with PVC, and therefore is used in manufacturing pipes, joints, industrial plates, sheets or the like where heat resistance is required.
However, CPVC is inferior in heat stability to PVC, and therefore, it easily undergoes scorching during molding processes, and hence is not suited for long time processing for molding. Discoloration troubles thus may be encountered in transparent moldings production, for instance.
Further, CPVC easily becomes electrified and therefore its free flow property is not good. For this reason, these resin tend to cause blocking of the hopper of an extruder, disturbing the intended constant extrusion, among other problems.
For these reasons, as a measure for resin floating, a technology was proposed in Japanese Laid Open Patent specification Kokai Hei 9-95513 (1997). The proposed method comprises chlorinating a PVC obtained by suspension polymerization in the presence of a specific amount of a specific polyvinyl alcohol as a suspending agent. This method indeed improves productivity since the floating resin problem can be avoided.
However, this proposed method is not yet free from the problems of heat stability in molding processes and/or discoloration of moldings, for transparent applications. In addition, the free flow property problem is still to be solved and, therefore, a constant extrusion operation is often difficult to achieve because of blocking of the hopper of the extruder, among other problems.
It is an object of the present invention to provide a novel process for the production of chlorinated polyvinyl chloride resin (CPVC), which prevents resin floating in the chlorination process and gives CPVC having excellent free flow property and further showing good resistance to initial discoloration as well as good heat stability at the time of heating and molding processing.